1. Field of the Invention
The present invention relates to a micro-electro-mechanical system (MEMS) device and a method of fabricating the same.
2. Discussion of the Related Art
Micro-Electro-Mechanical Systems (MEMS) are integrated sensors, actuators, and electronics fabricated with processes similar to those used for integrated circuits. They integrate mechanical elements (such as sensors and actuators) with electronics on a common substrate through microfabrication technology. They convert physical parameters to or from electrical signals, and depend on mechanical structures or parameters in important ways for their operation.
MEMS devices use a sensor with sensing circuitry and/or an actuation device with drive circuitry to detect or produce physical phenomenon. MEMS sensors gather information by measuring any combination of mechanical, thermal, biological, chemical, optical, and magnetic phenomena. Electronics then process the information derived from the sensors, and through some decision making capability can direct the actuators to respond. Non-limiting responses include moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose.
Early MEMS devices were used as accelerometers for automobile crash airbag deployment systems. Now, MEMS accelerometers are quickly replacing conventional accelerometers for crash airbag deployment systems in automobiles. The conventional approach uses several bulky accelerometers made of discrete components mounted in the front of the car with separate electronics near the airbag. MEMS made it possible to integrate the accelerometer and electronics onto a single silicon chip. MEMS accelerometers are therefore much smaller, more functional, lighter, more reliable, and are produced for a fraction of the cost of the conventional macroscale accelerometer elements.
MEMS can now be used in many other ways. Other non-limiting examples include pressure sensors, microvalves, and gyroscopes. They have applications in many areas, including health care, industrial automation (including automated semiconductor manufacturing), automotive systems (both vehicles and smart highways), global environmental monitoring, environmental controls, defense, and a wide variety of consumer products.
MEMS devices can also be used as switches in fiber optic networks. A MEMS optical switch includes at least one input port in optical communication with the proof-mass and at least one output port in optical communication with the proof-mass. The proof-mass directs light from at least one input port to at least one output port. When an electrostatic potential is applied to the at least one top and bottom electrodes, an electrostatic force is generated which causes the proof-mass to move and direct the light from at least one input port to at least one output port. The proof-mass then remains static until the light path needs to be redirected. In certain embodiments the proof-mass may form at least one mirror, at least one partially reflective mirror, and/or at least one diffraction grating. The proof-mass may be transparent to at least one wavelength of light. In other embodiments the device further contains at least one optical coating disposed on the proof-mass. The at least one optical coating may form at least one mirror, at least one partially reflective mirror, and/or at least one diffraction grating. The at least one optical coating can be conductive or non-conductive, and can be transparent to at least one wavelength of light. In certain embodiments the input and output ports may be fiber optic lines. These mirror-based switches can be two-dimensional, where they move up and down or left and right, or three-dimensional, where they can swivel in a broad range of movement. In other embodiments, the optical switch can be employed in an array, with up to thousands on a single chip. The result is an end-to-end photonic network which is more reliable and cost-effective, and has minimal performance drop-off. Additional applications include active sources, tunable filters, variable optical attenuators, and gain equalization and dispersion compensation devices.
MEMS devices are manufactured using batch fabrication techniques similar to those used for integrated circuits. Micro-mechanical components are fabricated using processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and/or electromechanical devices. MEMS devices may contain a sensor device that makes use of a proof-mass suspended structure, and sensing circuitry that is commonly formed using typical semiconductor-type fabrication processes. Exemplary MEMS manufacturing techniques are described in the following references, which are hereby incorporated by reference in their entirety into this application: John J. Neumann, Jr. & Kaigham J. Gabriel, CMOS-MEMS Membrane for Audio-Frequency Acoustic Actuation, 95 Sensors & Actuators A 175-82 (2002); M. Mehregany et al., Integrated Fabrication of polysilicon Mechanisms, 35 IEEE Transactions on Electron Devices 719-23 (1988); Huikai Xie et al., Post-CMOS Processing for High-Aspect-Ratio Integrated Silicon Microstructures, 11 Journal of Microelectromechanical Systems 93-101 (2002); Kaigham J. Gabriel, Engineering Microscopic Machines, 273 Scientific American 118-21(1995); Andrew A. Berlin & Kaigham J. Gabriel, Distributed MEMS: New Challenges for Computation, 4 IEEE Computational Science & Engineering 12-16 (1997).
Fabrication processes for existing MEMS devices are inefficient and costly due to the combination of individual steps required to fabricate a single device. Additionally, while electronic signal processing is increasingly being used in MEMS—in sensors, actuators, and integrated electronics, existing MEMS applications are limited in that they have relatively low levels of electromechanical integration and little interaction with mechanical components working alone or together to enable a complex action. For example, in a typical integrated circuit, the circuitry and the proof-mass suspended structure are formed on separate semiconductor layers. While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micro-mechanical components are fabricated using compatible “micromachining” processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices. Therefore, the integration of on-chip circuits and proof-mass suspended structures usually require additional deposited layers or are built on a separate die.
Accordingly, a need exists for MEMS products with greater levels of electrical-mechanical integration. To satisfy this need for increasing levels of integration in MEMS devices, monolithic chips or multichip modules need to be developed. These monolithic devices would integrate sensing, driving, controlling, and signal processing electronics into fewer layers on a substrate. This integration promises to improve the performance of micro-mechanical devices, as well as reduce the cost of manufacturing, packaging, and instrumentation for these devices, by combining the micro-mechanical devices with an electronic sub-system in the same manufacturing and packaging process.